1. Field of the Invention
The present invention relates to electrophotography. More particularly, the present invention is directed to a novel method and apparatus for electrophotography which enables one to selectively produce a positive-positive image at one time, negative-positive image at another time or both at the same time.
2. Description of the Prior Art
As electrophotography there are known and used various systems such as Electrofax, Xerox, P.I.P. (Persistent Internal Polarization) and NP system. For example, reference is made to U.S. Pat. Nos. 3,666,363 and 4,071,361 and German Pat. Nos. 1,522,567 and 1,522,568.
According to Electrofax and Xerox systems, an electrostatic image is formed in accordance with the principle of the well-known Carlson process as described in U.S. Pat. No. 2,297,691. The photosensitive medium used in these systems comprises a layer of photoconductive substance such as zinc oxide (Electrofax), amorphous selenium (Xerox) and the like formed on a substrate. The surface of the photoconductive layer is electrically charged by corona discharge and then it is exposed to an original image. At the area exposed to the light, the electric charge is decayed so that an electrostatic image corresponding to the light and dark pattern of the original is produced. The electrostatic image is visualized by developing it with charged and colored particles. The visualized image is fixed directly after development or after transferring it onto a supporting member such a sheet of paper. Thus, an electrophotographic image can be obtained.
P.I.P system is featured by the fact that a latent image is formed making use of particular physical properties of fluorescent material, that is, persistent internal polarization and photoconductivity. The NP system uses such photosensitive medium comprising further an insulating layer overlaid the above mentioned photoconductive substance layer. An electrostatic image is formed making use of the difference in electrostatic capacity between the two layers as well as the photoconductivity thereof. An electrophotographic image is obtained from such latent image through the steps of developing, transferring and fixing which are essentially the same as above.
Many types of copying apparatus have been developed based upon the above described various electrophotography techniques. Generally speaking the copying machine is used to produce images in a mode of positive-positive, namely to obtain a positive copy from a positive original. However, sometimes it is wished to obtain a positive copy from a negative original using the same copying machine for positive-positive. This applies, for example, to apparatus for enlarging and printing microfilms.
To attain the dual purposes of positive-positive and negative-positive with one and same apparatus it was hitherto required to change the polarity of electric charge applied to the photosensitive medium or to change the polarity of developing toner for two different cases. This makes the apparatus complicated, enlarged and very expensive. Moreover, images thus produced are unstable.
As a solution to the above problem, U.S. Pat. No. 3,990,791 has disclosed an image forming apparatus for forming positive and negative images employing the known electrophotographic technique.